The use of thermal insulative packaging for the shipment of frozen or refrigerated items has broad applications in present-day commerce. Applications throughout the food, biomedical, clinical sampling, and industrial manufacturing markets require the use of an insulated transport device to keep samples cold for extended periods of time. Maintaining cold temperatures, while shipping products, has been done using three basic methodologies: 1) employing a powered refrigeration unit, 2) through contact with cold material such as ice or solid carbon dioxide (dry ice), or 3) using the "coldness" of the product itself. While a refrigeration unit permits storage times of years, storage times using ice or dry-ice is commonly less than a week, and storage times of less than a day can be expected when employing the "coldness" of the product itself.
On a broad scale, the cost of using each methodology is proportional to its storage time capability. In addition, selection of a methodology for storing an item is also dependent on the quantity or number of items being shipped. While a refrigeration unit is more appropriate for bulk shipments over long distances, use of a cold pack is more appropriate for an overnight shipment of a single product.
The efficiency and efficacy of each storage method is most affected by the insulative characteristics of the container itself, or, in other words, the barrier that each container presents to external heating elements. Heating of a product occurs in three ways: conduction, convection, and through radiation. The successful insulation of a cold product inside a container is solely dependent on the ability of the container to inhibit these three heating factors. In general, when designing the container so as to meet the insulative standards desired, it is necessary to understand how each heating element can most effectively be countered.
In understanding the underlying problems, it is useful to consider the effects of each of heat conduction, convection, and radiation.
Conduction
Conduction occurs as heat, in the form of molecular vibration, passing from molecule to molecule through a material. The conductive heat flow, Q.sub.c, through a material of thermal conductivity, k, area, A, and temperature gradient across the material, .DELTA.T is: EQU Q.sub.c =(k)(A)(.DELTA.T) Eq. 1
Typical k values, in units of Btu/(h.multidot.ft.sup.2.multidot. .degree.F.multidot.ft), for various substances are given below:
TABLE 1 ______________________________________ Substance Temperature (.degree.F.) Value ______________________________________ Air 32 0.0140 Aluminum 70-700 130 Argon 32 0.00915 Carbon Dioxide 32 0.0084 Gold 60-212 196 Polystyrene 32 0.021 Silver 70-600 242 Sulfur Dioxide 32 0.005 Water 32 0343 ______________________________________
In choosing container design parameters so as to minimize the heat conduction through the container and therefore maximize insulation (regardless of temperature difference between the environment and the inside of the container), one would clearly choose to use a material with a minimum k factor (such as sulfur dioxide), with infinite wall thickness, and of spherical form (where A is smallest for the internal volume). In practical terms, though, one must consider material costs, shipping costs, and structural integrity when designing the container. To date, the most common container for shipping chilled items using ice or dry-ice is a hollow expanded polystyrene (k=0.02) box of wall thickness commonly ranging from 1 to 3 inches.
Convection
Convective heat transfer occurs between two surfaces at different temperatures that are separated by a free-flowing fluid or gas. As an example, consider placing a pot filled with water atop an electric stove. As a unit of water is heated at the bottom surface of the pot, its density diminishes with respect to the surrounding water, and it therefore rises to the top of the pot, touching the cold surface of the pot cover. Upon touching the cover, the unit of water transfers its heat to the pot cover, thus cooling down, increasing in density with respect to the surrounding water, and further sinking back to the bottom of the pot. This cycling creates convective heat currents between the hot bottom and cold top of the pot.
In regard to keeping items cold (or warm) within an insulative box one must minimize the convective heat transfer that occurs between the cold product and the environment. The Grashof criterion is used to determine whether convective heat transfer will occur between walls at different temperatures filled in-between with a given filler medium. For blocking convective heat transfer between given walls of the container, the Grashof criterion must be less than one thousand. ##EQU1## Where: g=gravitational constant (m/sec.sup.2)
l=distance between walls (m) PA1 v=viscosity of fluid between walls (m.sup.2 /sec) PA1 T.sub.1 =temperature of insulated objet (.degree.C.) PA1 T.sub.2 =ambient temperature (.degree.C.) PA1 .epsilon..sub.1 =Emissivity of the object PA1 .alpha..sub.1 =Absorptivity of the object
Using Grashofs criterion, one can determine the minimum distance between walls, 1, for which convective heat transfer will not occur. Using air, an ambient temperature of 30.degree. C., and an interior sample temperature of -10.degree. C., a minimum spacing of 0.495 centimeters between air gaps in an air-filled wall are necessary to prevent convective cooling. An expanded polystyrene or similar material box has walls that are made of closed cells containing air. The box acts as an effective barrier to thermal convective heating since the diameter of each cell measures far less than Grashof's threshold value of 0.495 centimeters. The primary problem with using a polystyrene box as an insulator, however, lies in its inherent bulk and lack of collapsibility during storage. Secondary environmental problems in using polystyrene involve the current lack of a diffuse recycling program for the material as compared to recycling programs for low and high density polyethylene.
The use of honeycomb or cellular structures inside walls is also widely discussed in the prior art. These structures are intended to be permanently affixed on the inner space between an exterior wall of a sheltered structure (such as a home or building) and the interior wall of the structure. Among prior art proposals are those of U.S. Pat. Nos. 3,314,846; 3,547,751; 4,673,600; 4,865,889; 5,062,751; 5,171,114; and U.S. Pat. No. Re. 26,444. Although such structural approaches could be adopted for minimizing convective heating of a cold sample during shipment, the cost of creating a container that incorporates the baffles is, to date, inherently prohibitively expensive.
In addition, many of these baffled structures are commonly made of paperboard material, which must be treated to avoid disintegration from contact with moisture commonly forming near a cold object through condensation of water from air. U.S. Pat. No. 2,703,770 describes a honeycomb structure created using plastic material and alternating heat sealing dots. The process for the use of such an approach, however, is extremely slow as the rate of the machine is limited by the inherent time required for heat sealing the dots. While increased rates may be achieved through multiple heat sealing fixtures, such prove expensive and difficult to assure proper quality control.
Radiation
Radiative heating of a body is most commonly observed as infrared radiation from the sun striking an object and, depending on the emissivity and absorptivity of the object, raising its temperature. Radiation is emitted not only from burning stars, but from all surfaces; the level and spectrum of the radiation being solely dependent on the temperature of the surface.
In considering a body which absorbs all incident external radiation and emits radiation solely as a function of its absolute temperature T, also called a blackbody source (with emissivity .epsilon.=1), the radiative heat Q.sub.R from the body is given by the Stefan-Boltzmann law: EQU Q.sub.R =.epsilon..sigma.T.sup.4 Eq. 3
Where: EQU .sigma.=Stefan-Boltzman constant=5.67.times.10.sup.-8 W/m.sup.2 (K).sup.4
In considering an object or package of area A.sub.1 at absolute temperature T.sub.1 surrounded by a blackbody at absolute temperature T.sub.2 with emissivity, .epsilon..sub.2 =1, as may be characterized by the surrounding walls of a room or a closed truck, the radiant heat exchange between the object and the surroundings is given by the following: EQU Q.sub.1-2 A.sub.1 .sigma.(.epsilon..sub.1 T.sub.1.sup.4 -.alpha..sub.1 .epsilon..sub.2 T.sub.2.sup.4) Eq. 4
Where:
While the difference in temperature between object and surroundings affects heat transfer from conduction and convection in Eqs. 1 and 2 in a linear mode, radiative heat transfer changes with the 4th power of temperature difference in Eq. 4. For this reason, the radiative heat transfer properties of a package must be carefully studied as well as the ambient temperatures in which the package will be exposed.
In designing a package most effectively to block thermal heating from the environment, for example for shipment of frozen products, it is desirable to use a material with a low coefficient of absorptivity and a high coefficient of emissivity.
As before stated, for the purposes of the invention, the thermal insulation must also provide appropriate cushioning protective packaging properties.
Cushioning
The benefits of inflatable packaging are largely discussed in my co-pending application Ser. No. 092,750, filed Jul. 16, 1993 for Inflatable Flat Bag Cushioning and Method of Operating and Making The Same, in which there is disclosed an improved adjacent T-chamber, balloon or thin film flexible envelope packaging system, inflatable, for example, by injecting air simultaneously into the envelope chambers through a single inflation inlet. The inlet is provided with a self-sealing flutter valving mechanism, such as that described in my further co-pending application Ser. No. 278,610 filed on Jul. 21, 1994 for Flutter Valve Assembly For Inflatable Packaging And The Like, enabling independent chamber filling and sealing; and such also being also deflatable to permit reuse of the envelopes.
Such inflatable packaging structures and the like, provide adequate cushioning for fragile articles by using large pockets of compressible medium, such as air, to surround the object. In the case of using a medium such as air, it is necessary that these pockets be large so as to provide the needed cushioning characteristics. Unfortunately, though, as is discussed in the convection section above, these pockets also provide excellent regions for convective currents to form that bring the interior object to the ambient temperature.